DNA topoisomerase II and I can be purified to homogeneity from mouse L1210 and Chinese hamster (DC3F; DC3F/9-OHE) cells. The purification procedure is based upon the detection of topoisomerase II by its DNA linking and breaking activity in the presence of m-AMSA. Purified topoisomerase II produced protein-associated DNA breaks in the presence of the same compounds that produce protein-associated DNA breaks in cells. In addition, in both cases the breaks do not allow DNA swivelling, their 5'-termini are covalently linked to the enzyme and their formation can be inhibited by high concentrations of 2-methyl-9-hydroxyellipticinium. It is therefore likely that topoisomerase II is the intracellular target of m-AMSA, anthracyclines, ellipticines and epipodophyllotoxins, by the intermediate of which they produce the protein-associated DNA breaks. Cyanomorpholinoadriamycin has a unique mode of interaction with DNA involving DNA interstrand crosslinks and no (or weak) antitopoisomerase II activity. m-AMSA seems to interact directly with topoisomerase II by the intermediate of its side chain. DNA intercalation is not sufficient to affect topoisomerase II. For example, 9-aminoacridine is a better intercalator than m-AMSA but has no (or weak) antitopoisomerase II activity. Polyamines affect topoisomerases activities and could play a role in controlling DNA topoisomerases in mammalian cells.